Tons of radioactive water were discovered on Tuesday to have leaked into the ground from Japan’s Fukushima nuclear plant, the latest in a series of leaks at the plant damaged in a March earthquake and tsunami, the country’s nuclear watchdog said.

More than three months after the disaster, authorities are struggling to bring under control damaged reactors at the power plant, 240 km (150 miles) north of Tokyo.

About 15 metric tons of water with a low level of radiation leaked from a storage tank at the plant on the Pacific coast, the Nuclear and Industrial Safety Agency said.

Plant operator Tokyo Electric Power Co (Tepco) said it was investigating the cause of the leak which was later repaired.